In the manufacture of semiconductors, semiconductor devices are produced on thin disk-like objects called wafers. Generally, each wafer contains a plurality of semiconductor devices. The importance of minimizing contaminants on the surface of these wafers during production has been recognized since the beginning of the industry. Moreover, as semiconductor devices become more miniaturized and complex due to end product needs, the cleanliness requirements have become more stringent. This occurs for two reasons.
First, as devices become miniaturized, a contaminating particle or impurity on a wafer will occupy a greater percentage of the device's surface area. This increases the likelihood that the device will fail. As such, in order to maintain acceptable output levels of properly functioning devices per wafer, increased cleanliness requirements must be implemented and achieved.
Second, as devices become more complex, the raw materials, time, equipment, and processing steps necessary to make these devices also become more complex and more expensive. As a result, the cost required to make each wafer increases. As such, in order to maintain acceptable levels of profitability, it is imperative to manufacturers that the number of properly functioning devices per wafer be increased. One way to increase this output is to minimize the number of devices that fail due to contamination. Thus, increased cleanliness requirements are desired.
Semiconductor wafers are typically processed in a process chamber, either as a single wafer or in batches. During production, the semiconductor wafers are subjected to a number of wet processing steps, such as cleaning, rinsing, drying, etching, etc., that expose the wafers to various process fluids, such as deionized (DI) water, acids, diluted chemistries, and gas-liquid solutions. These fluids often carry contaminants, such as particles and ionic impurities. Thus, the process fluids themselves can act to contaminate the wafers and decrease yield by carrying contaminants to the wafer surfaces.
One way in which particles and other impurities can be introduced into the process fluids is from the pipes carrying the fluids from their reservoirs/sources to the process chamber. For example, in a DI water distribution system, such as in a semiconductor foundry, DI water is typically purified from the local water supply at a central facility and then supplied to wet benches throughout the foundry by a distribution network. Even if pure water leaves the central facility, bacteria growing in the pipes can enter the rinsing system, and contaminate the wafers. In addition, the inner surface of the piping itself can introduce particles. Sources of contamination for example, can also include valves, piping elbows, and other moving parts. Thus, even if the rinse water is typically filtered by the DI water making plant, the particle counts are still high enough to negatively impact the quality of the wafer cleaning results because these liquid-borne particles will deposit onto the wafers when exposed to the DI water.
Operating a clean air and water distribution networks throughout the foundry presents an economic problem. Different parts of the manufacturing process place different demands on the cleanliness of the air and water. For example, post-HF wet processing requires extremely clean air and water, while other operations may be less susceptible to particle contamination. The cost of operating a cleaning facility is related to the degree of cleanliness and the volume of water required. Using only a centralized purification system requires that all of the air and water purified for all of the foundry that meet the requirements of the most demanding operation. Because only a relatively small amount of air and water needs to be of the highest purity, this may result in an unduly expensive central water purification system.
Additionally, even if DI water plants usually produce ultra pure DI water some contamination still takes place, for example, during the change of the resin beds. Due to the low ionic strength and high pH of DI water, these impurities can transfer from the DI water to the silicon surface of the wafer during processing. Even at 50 ppt, contamination levels in DI water can result in a measurable E10 atoms/cm2.
To help eliminate contaminants that develop in the water distribution system relying on centralized purification, the entire system may be purged. For example, an H2O2 purge and DI water flush can remove bacteria growing in a DI water system. In such a centralized system, however, wafers cannot be produced while the system is being purged, causing production delays and shutdown of some operations. Consequently, such steps may be taken only when contamination problems become known to the operators. This exposes the wafers to significant levels of impurities in the water between the system purges.
Process fluid contamination problem also arise in process chambers utilizing recirculating systems. In such systems pumps and any other moving parts in the loop can be the major contributors of contamination. Moreover, because the same process fluid is being recirculated back into the process chamber after contacting (and processing) wafers, contaminants, including particles, metals, photoresist, etc., that are removed from the wafers can be reintroduced back into the process chamber and contaminate the wafers.
A number of systems have been developed in an attempt to alleviate these problems, for example, a point-of-use filtration system is disclosed in U.S. Pat. No. 6,491,043 ('043), Mohindra et al., which is hereby incorporated by reference in its entirety. The Mohindra et al. system disclosed in the '043 patent utilizes a filter bank provided on a fluid supply line prior to the process chamber. The filter bank comprises an ion exchange module and a combination of charged and neutral filters. However, the Mohindra et al. system is less than optimal for a number of reasons.
First, the filter bank of the Mohindra et al. system only removes contaminants that are about 0.1 microns (μm) or larger. With the devices on wafers becoming even more miniaturized, contaminants that are smaller than 0.1 m can negatively impact yield. Thus, the Mohindra et al. system will not meet industry needs.
Moreover, the filter bank of the Mohindra et al. system disclosed in the '043 patent does not sufficiently remove metallic ionic impurities from the fluids. In order to reduce and/or eliminate such metallic impurities from the fluid, the Mohindra et al. system utilizes an injector to inject hydrochloric acid into the fluid and/or fluid lines. The process and apparatus required to inject chemicals/acids into the fluid stream to remove metals have the disadvantage of containing moving parts, inaccurate dispense, and possible malfunctioning.
Finally, the Mohindra et al. system does not contain any means by which the contamination levels of the process fluid (e.g., DI water) can be monitored. As such, the process fluid may contain unacceptable amounts of contaminants and the user may continue to process wafers until yield issues and/or defects in the devices come to light somewhere along the production line. This can expose many wafers to unfavorable processing conditions resulting in many wafers being ruined, thereby resulting in substantial economic detriment.
Thus, a need exists for an improved system and method of providing ultra-pure fluid to a process chamber for the processing of substrates.